LEDs are a desirable choice of light source in part because of their relatively small size, low power/current requirements, rapid response time, long life, robust packaging, variety of available output wavelengths, and compatibility with modern circuit construction. These characteristics may help explain their widespread use over the past few decades in a multitude of different end use applications. Improvements to LEDs continue to be made in the areas of efficiency, brightness, and output wavelength, further enlarging the scope of potential end-use applications.
LEDs are typically sold in a packaged form that includes an LED die or chip mounted on a metal header. The header can have a reflective cup in which the LED die is mounted, and electrical leads connected to the LED die. Some packages also include a molded transparent resin that encapsulates the LED die. The encapsulating resin can have either a nominally hemispherical front surface to partially collimate light emitted from the die, or a nominally flat surface.
Examples of known LED sources are given in FIGS. 1-5.
In FIG. 1, an optical semiconductor device 10 includes a support 11 and an electroluminescent semiconductor diode 12 secured to a top surface of support 11 by a suitable solder. Terminal wires 13, which extend through openings in the support 11, are secured to and electrically insulated from the support by washers 14 of an electrically insulating material, such as glass or ceramic. Each terminal wire 13 is electrically connected to a separate contact of the diode 12 by a fine wire 15. A third terminal wire 16 is secured to the support 11 which is electrically connected to the diode 12. A glass dome 17 is mounted on and secured to the top surface of support 11. The glass dome extends over and is in intimate contact with the diode 12 so that radiation emitted by the diode passes through the glass dome. In device 10 of FIG. 1, the glass dome is nearly spherical in shape. FIG. 2 shows an optical semiconductor device 10′ similar to device 10 except that the glass dome 17′ of FIG. 2, which is mounted on the support 11 and covers the diode 12, is hemispherical in shape. The devices of FIGS. 1 and 2 are described further in U.S. Pat. No. 3,596,136 (Fischer). For example, Fischer discloses elliptical, parabolic, and other desired shapes for the glass dome to convey radiation from the diode to a desired receiver in an efficient manner. Fischer also teaches that the glass dome 17 or 17′ is, among other things, made of a glass having a high index of refraction, preferably greater than 2 and as close as possible to the index of refraction of the electroluminescent diode, and of a low absorption.
In FIG. 3, a semiconductor light-emitting device 18 is shown. The device 18 includes a semiconductor chip 19 having a narrow light emanating region 20. A spherical lens 21 is mounted on a rear surface 22 of the chip with an ultraviolet hardening adhesive 23. The semiconductor chip 19 is obtained by scribing and dividing a larger semiconductor wafer, and the substrate side of the wafer (later forming the rear surface of chip 19) is ground with high precision to a predetermined thickness such that the distance between the center of the lens 21 and the center of the emanating region 20 is optimum. The device 18 is described further in U.S. Pat. No. 5,403,773 (Nitta et al.). Nitta et al. state that the refractive index of the adhesive resin 23 should be similar to the refractive indices of the device chip 19 and the spherical lens 21.
In FIG. 4, an LED-excited phosphor-based light source 24 includes a semiconducting LED 25 mounted in a well of an electrically conductive heat sink 26 that also reflects some of the light emitted from LED 25 toward a phosphor-reflector assembly 27. The assembly 27 can reside in an optically transparent potting material 28 which can be shaped to provide a lens feature 29 to tailor the light emitted by the light source 24. The source 24 is described further in U.S. Application Publication US 2004/0145288 A1 (Ouderkirk et al.).
In FIG. 5, a multi-layer LED 30, discussed more fully in U.S. Pat. No. 6,717,362 (Lee et al.), includes a semiconductor material 31 forming a p-n junction diode, the material 31 being encapsulated by three layers 32, 33, 34. The materials of layers 32, 33, 34 are chosen such that the refractive indexes of the layers progressively reduce, from close to that of the semiconductor material 31, to close to that of air 35. According to Lee et al., this results in small refractive index differences between the respective interfaces of the semiconductor material 31 and the first layer 32, the first layer 32 and the second layer 33, the second layer 33 and the third layer 34, and finally the third layer 34 and air 35. Lee et al. teach that any number of layers may be used, as long as the refractive indexes of the layers have the above properties.